South Korea said Thursday that an international investigation has found overwhelming evidence that one of its warships was sunk by a torpedo made in North Korea and that the weapon was fired by a North Korean submarine.

"There is no other plausible explanation" for the sinking of the Cheonan on March 26 near a disputed sea border between the two Koreas, said the report. It was based on the findings of 50 experts from South Korea who worked with 24 investigators from the United States, Australia, Britain and Sweden.

North Korean denied Thursday that it had attacked the ship, saying it wanted to send its own investigators to Seoul to examine evidence. In an unusually quick response to the investigation, the North said the findings were fabricated. It also threatened to respond to even a minor retaliatory attack with a "strong physical blow that knows no mercy."

But the investigation, details of which have been dribbling out this week, is certain to lead to a sustained effort by South Korea, the United States and Japan to seek punitive action against North Korea by the U.N. Security Council.

The report said that investigators had confirmed that "a few small submarines and a mother ship" had left a North Korean naval base two to three days before the attack on the Cheonan and returned to port two to three days later.

It also found that torpedo parts recovered at the explosion site "perfectly match" schematics of a North Korean-made torpedo that the Pyongyang government has offered to sell to foreign countries.

The exploded torpedo was fired by a midget submarine, equipped with night-vision technology, that approached the Cheonan from international waters to ensure "the covertness" of the attack, South Korean Rear Adm. Hwang Won-dong said Thursday. Speaking at a news conference held to elaborate on the findings, he said the midget submarine fled the scene following the same route.

Investigators collected parts that were marked with Korean writing, the report said, and the markings were "consistent with the marking of a previously obtained North Korean torpedo," which allowed the investigators to "confirm that the recovered parts were made in North Korea."

Parts from the recovered torpedo were shown in a nationally televised presentation, as South Korean Army Brig. Gen. Yoon Jong-seong explained how the parts exactly match blueprints of a North Korean-made torpedo. The torpedo's propeller assembly was discovered May 15 in a dredging operation that took place near the site where the Cheonan sank.

South Korea and American experts said at the news conference that the findings of the investigation were unanimous.

In Washington, Deputy Secretary of State James B. Steinberg said Wednesday that the United States and South Korea have "worked very closely" on the investigation. He called the investigation a "very professional, very methodical effort to understand what happened . . . to follow the facts where they lead."

South Korea is seeking to use the investigation to garner broad international support for U.N. sanctions against North Korea and to persuade China to support them.

China, which is North Korea's primary patron and largest trade partner, has been skeptical. Chinese Foreign Minister Yang Jiechi told the foreign ministers of South Korea and Japan last weekend that any conclusions about the sinking must be based on scientific and objective evidence.

On Wednesday, South Korean Foreign Minister Yu Myung-hwan told European diplomats and business leaders in Seoul that efforts to punish North Korea "will not have much effect without the concerted efforts of the international community."

Still, there is considerable doubt even in the South Korean Foreign Ministry about how far Seoul will be able to go with its allegations against the North, given China's veto in the U.N. Security Council.

There is a rising tide of irritation in South Korea at China, which waited nearly a month to voice sympathy for those killed in the sinking.

"The level of South Korean anger against China is the highest I have ever seen," said Brad Glosserman, executive director for the Honolulu-based Pacific Forum of the Center for Strategic and International Studies.

He described the sinking of the warship as a "major formative experience" for a new generation of South Koreans as they deal with the unpredictable military threat posed by North Korea.